Method of preparing phosphorous acid



April 29,

1 J. w. LEFFORGE ET AL METHOD OF PREPARING PHOSPHOROUS ACID Filed Sept. 25, 1948 HCI l :o

JOHN w. LEFFORGE ROBERT B. .HUDSON INVENTOR.

ATTORNEY i atented Apr. 1952 METHOD OF PREPARING PHOSPHOROUS ACID John W, Lefforge and Rober'tB. Hudson, Anniston, Ala., assignors toMonsanto .Chemical Com.- pany, St. Louis, Mo., a corporation of Delaware Application September 25, 1948, Serial No.*51 ,1'94

The present invention relates to "phosphorous acid,.salts thereof. and to ainovel method ofmanufacturing same.

One object of the invention is to provide an economically and commercially .feasible method of producing phosphorous acid and the alkali metal salts thereof.

Another object of the invention is to provide .a continuous method of producing the foregoing products.

An .additional object is to provide a direct method of producing highly concentrated phosphorous acid by hydrolysis of phosphorus trichloride-in which excessive losses of the latter by volatilization are obviated.

A :further object is to providea method of producing phosphorous acid .by hydrolysis of phosphorus trichloride in Whichsubstantial decomposition of the reactants and/or product into undesired products is avoided.

Astill further object is to provide a method of producing phosphorous acid by hydrolysis of 9 Claims. (01. 23 1ss phosphorus trichloride wherein a product is..ob-

tained which isfree or. substantially: free of hydrogen chloride.

It is also an object of the instant invention to provide a newsalt of phosphorous (acid, namely hemisodium phosphite: NaHzPQaI-IsPOs.

other objects andadvantages of thezinvention will be obvious to those skilled in the art as the description proceeds.

*Heretofore, several methods of hydrolyz-ing phosphorus trichloride to form phosphorous acid have beenemployed and while they have :been satisfactory for laboratory use, theyi'have'not been adopted, so. far as we are-aware. on a commercial scale.

.For example, it is known that phosphorous. acid may be prepared by reactingtogether at-1ow temperatures.stoichiometric amounts of phosphorus trichlorideand water. but as previously practiced, this method results in substantial decomposition of the desired product and/orexcessive loss of phosphorus trichloride. by volatilization.

.Asa means 'of overcoming these disadvantages, it has been proposed to moderate the reaction by the combined use of low. temperatures and a large .excess of water.

7 This technique results in the substantial elimination of side reactions, excessive decomposition and loss of .phosphorus triichloride; but the use of a large excess of water yields'dilute phosphorous acid which must be concentrated ifsuch a "product is desired.

As a further modification of the above method,

.cold concentrated hydrochloricacid has been 2 used to? moderate the hydrolysisreaction and at the same time avoid excessive dilution ofthe product. but .the phosphorus trichloride losses .are stillyso excessive as to render the method commercially unattractive.

Thus, according to the prior art, low'temperatures and concentrated hydrochloric :acid are essentialto the-production of concentrated ph sphorous acid with .aminimu-m loss of phosphorus trichloride by ,volatilization and/or loss of; product by decomposition. However, contrary to this conception, we have ,made the surprising discoverytha the abov obj ctives anvbe a i ed by reacting phosphorus 'trichloride with water or phosphorous acid at relatively high temperatures, that is, at temperatures equivalent to or above the boiling point of the trichloride. Moreover, we have discovered that contrary to expectations, phosphorus trichloride hydrolyzes readily in concentrated phosphorous acid and that the latter may be used as ahydrolyzing medium to directly produce more highly concentrated acid without the necessityof resorting to separate concentrating operations.

Stated .in its broadest aspect, the. present invention comprises reactingphosphorous trichloride with wateror phosphorous acid at a temperature equivalent to or. above the boiling point of phosphorus trichloride, :but @below that temperature at whichsubstantial decomposition of; the

trichloride into products other than phosphorous acid takes place.

M sp fical y, the inven ion is n n d with a method. of directlyproduc-ing. practically 1 00% phosphorous acid without. substantial loss of P013, which method involves 'hydrolyzing the ously flow countercurrently to each other.

Expressed in a stillmorelimitedform, the invention comprises-producing phosphorous acid by introducing phosphorus trichloride-intoa suit- 1 able tower containing a heel of phosphorous acid of about to strengthunder conditions insuring at least 66 hydrolysis in the heel, passing the volatilized PClz-HCI mixture tip-through the tower countercurrentlytoa stream of water or aqueous phosphorous ,acid to hydrolyze the .unreacted Pcla withdrawing the crude concentrated phosphorous .acid from the iheel, as, rap- 3 idly as it is formed in the tower, dechlorinating and then neutralizing the resulting product.

The hydrolysis of phosphorus trichloride to phosphorous acid may be carried in systems of various types. 7

For example, the hydrolysis reaction may be effected in a continuous manner by passing phosphorus trichloride into the bottom of a spraypackedor bubble-cap-tower countercurrently to the flow of water or phosphorous acid. In such a system, the product is continuously withdrawn from the bottom of the tower and the hydrogen chloride produced as a by-product is continuously exhausted from the top thereof. This system may be modified by providing means for recycling the phosphorous acid to the top of the tower where it may be reintroduced, with or without the addition of water, for further reaction with phosphorus trichloride. The phosphorous acid thus obtained is repeatedly recycled in whole or in part until the desired concentration is obtained. 1

The reaction may also be executed in a cascade system of relatively small reactors wherein the phosphorus trichloride flows from one to the other until completely reacted and the reaction liquors of successively lower concentrations of EPO: are conveyed in a countercurrent direction from one reactor to the other until the phos-- phorous acid is concentrated to the desired degree.

Another system in which the hydrolysis may be carried out is one including a series of small reactors in which the reaction liquors are successively pumped from one to the other in a direction countercurrently to the flow of phosphorus trichloride.

A further system which may be employed is one wherein a substantial part of the hydrolysis takes place in a liquid reaction zone and the unreacted phosphorus trichloride is then removed from the volatilized HCl-PCh mixture by scrubbing with water or phosphorous acid in a separate vapor reaction zone.

The preferred system, however, is one involving the use of a packed tower containing a heel of phosphorous acid. In accordance therewith, phosphorus trichloride is introduced into the heel under conditions resulting in hydrolysis of at least 66% of the trichloride and the remaining unreacted PCla volatilized with the hydrogen chloride produced is passed up into a vapor section where it meets a descending stream of water or phosphorous acid and is substantially completely converted to phosphorous acid. The undissolved hydrogen chloride and traces of phosphorus trichloride pass out of the top of the column and the phosphorous acid formed flows into the heel, from which product is discharged as rapidly as it is produced in the tower. This system has the advantage over those described above in that smaller and/or much less equipment is required for large volume production.

The foregoing systems are merely illustrative of those which may be used in practicing the hereindescribed method of making phosphorous acid, and it is to be understood that the invention is not restricted thereto as any gas and liquid contact system may be employed which provides a sojourn time, at the selected operating temperature and feed rates, of sufficient duration to efiect substantially complete hydrolysis of the phosphorus trichloride charged.

For a more complete understanding of the present invention, reference is made to the accompanying drawing which forms a part of this specification. This drawing shows a vertical cross sectional view of a suitable reactor for practicing the instant method, it being understood that the invention is not restricted thereto.

Reference character I represents the bottom or liquid section of the reactor in which a heel of phosphorous acid is introduced up to the level 2 indicated in the drawing. This section is provided with a PCls-inlet tube 3, an HaPOi discharge tube 4, packing rings 5 of glass, porcelain and the like and an electrical winding 6 for heating the liquid section to the desired temperature. An equilibrium, the zone below the P013 inlet tube becomes highly supersaturated with HCl and serves as a dechlorination section, heat being applied thereto by coil 6 to facilitate the liberation of HCl.

The center or surge bulb section of the reactor is represented by reference character 1, which section is provided to facilitate maintainil'lg the liquid at a constant level throughout the reaction.

The upper or vapor section is represented by reference character 8. This section is equ pped with a water inlet tube 9, and HCl discharge tube It), glass or porcelain rings II and a suitable heating coil l2. 7

In the practice of the present invention, liquid phosphorus trichloride is charged to the reactor at a rate designed to hydrolyze at least 66% of the trichloride in the liquid section, the remainder being volatilized with the liberated hydrogen chloride. The resulting gaseous mixture passes up through the vapor section where it is scrubbed by a descending stream of dilute phosphorous acid. The enriched scrubbing liquid continuously replenishes the heel in the liquid section from which the product is continuously discharged by way of outlet tube 4. The water used to produce the dilute phosphorous acid scrubhing liquid is continuously supplied in substantially stoichiometric proportions at the top of the vapor section by inlet 9, and the liberated I-ICl discharged therefrom by way of outlet tube The following example illustrates in greater detail the method of practicing the invention in the above apparatus:

Example I Phosphorus trichloride was continuously charged over a period of one hour and twenty minutes into a heel of 300 grams of approxi mately 89.6% HsPOs at a rate of about 26.31 grams per minute. Simultaneously therewith, water was continuously and countercurrently charged to the vapor section in substantially the theoretical proportions required to react with the P013 charged. The unreacted PCls in the liquid section was continuously volatilized into the vapor section along with the liberated HCl, where it was converted into additional quantities of HCl and phosphorous acid by reaction with the descending stream of water (or dilute phosphorous acid). p

The phosphorous acid produced in the vapor section entered the liquid section and continuously replenished the phosphorous acid heel, from which 99.4% HsPOa was continuously discharged from the system by pipe 4. gen chloride liberated as a result of the hydrolysis reaction passed up through the vapor section together with small amounts of water vapor, and the resulting mixture was continu- The hydro 5;; ously dischargeddrom-thesystemby wawof cut lt ituhelfli During the -reacti-on; the tempera'tures 1 in the liquid and vapor sections were controlledso that the vaporous mixture leaving the vapor section was' -at a temperature of about 9590. while the; product leaving the liquid section was at a temperaturezof'aboutl04 C. to 113" CI The =temperature of the reactor at thepoint of Pclzintrodiiction varied from-68! Cxto -=95-" Ci, but within about 5 minutes afterinitiation ofth'e reaction; the temperature' was-maintained at a value? equivalent: to or* abve= th&bbi1ing point (76 C5); of-p'hosph'orus trichloride andduring the greater part' off the reaction time, the temper-" ature was held within the range of about 86? C. tbi-aQOiCZ Approximately 83 to 8'7 of the phosphorus trichloride charged to the reactorwashydrolyzed inith'e liquid' section and except" for about 052 toL 0;75%" whichwas' lost and unaccounted for; theiremaining P013 was hydrolyzedin the vaporsection; The overall product yield in the two sections'was about 99.5% of theory.

The phosphorous acid produced by the above described method usually contains from about lv%2tOIlabOl1t15%i by weight of HCl'and'it is desirable-:lto-remove this impurity prior to storage oniprior toconverting the acid to the correspond ingsalkalihe salts; This is accomplished by heat-- ingithe crudephosphorous acid to a temperature of; 100C. to 150 C. while under -a* vacuum of approximately :30 mm; of mercury, but it is pref erably-achieved bypassing air or another suit-'- ableinertgas 'therethrough for a-period of about 30':minutes while heating sameto a temperature ofia'bout 150 C. These dechlorination methods are merely illustrative of techniques which may betemployed; and it is to be understood that any other HClremoval method which does not adversely. afiect the phosphorous acid product is within thescope of the invention.

Afterrremoval of the above impurity, the phosphorous? acid may be neutralized with various alkaline materials to form the corresponding saltss ForJexample; the essentially anhydrous salts of phosphorous acid: NaHzPOs, NazHPOs and NaI-IzPOa.HsPO3 are prepared by reacting to-- getherisubstantially stoichiometric proportions 'of molten: orv solid H3PO3 and sodium hydroxide; Therfproducts of thisireactlon are -free fiowing: crystallinewhite'powders: v Thehydrated saltszi Na2HPO3:5H2O" and NaHPOs':2.5HO are "prepared bycarrying out the reaction -in the foregoir'ig manner and then crystallizing the reaction product from aqueous solution at temperatures below- 100" C. and 40 C. respectively.

The hemisodium phosphite (NaH2PO3.H3PO3) referredto above may also be prepared in the following manner:

16.31 gramsof sodium' hydroxide melted up in* 10.=mil1ilitersiof water was'a'dded dropwise to 25' milliliters of784.81%: HsPOs, whereupon 20 milliliters of -the-resulting solution was concentrated atreduced pressure-and cooled. To the cooled concentrated" solution; 20 millilit'ers'of" 848% I-I3PO3 was introduced and the resulting solution cooledtoabout 30' C. The*crystalline= product thus obtained was filtered from its mother-liquor and then-Washed with methanol;

10' milliliters"of"84.'8% HaPOs wasadded .to theseparatedmother liquor and 1 the temperature third-prop of crystals produced. This product:-

wascseparatedby filtration 'and then washediwtthi methanol.

On:- analysis: the crystallinetproduct'sr: obtained in-=the-=above 'mamierwereefoun'd tozhavettliertolie: lowlngcompositiorr'.

Fem'e'n/t: 16. 67 87.8 u 16.78: 1

FoumiQLlIIIIlIiIIIl NilHzPoin NaHaPOa.2:5HgO H r 2 N nzroamrouhn t NSI 'H'PC; a- NSJHPOabHaO.-.

In additionato the above difierences; hemi'f-'- sodium phosphite is much more stable andless hygroscopic than phosphorous acid andthe other sodium phosphitesalts-mentioned above; I

Hemisodium' phosphite is a crystalline essene" tially non-hygroscopic solid which melts at about 156C. watersolutions of this saltare strongly acid (1%. solution: pH 1.8) and-havethereducing properties of "phosphorous: acid; It" cone" tains 'theequivalentpf 88 H3PO3 and maybe used as a" substitute for" concentrated phos= phorous acid which ishy-groscopic'and inconvenient to handle. It may aljsofbe ;us ed as1- a leavening acid;

The dechlorinating and neutralization steps have" been 'described above; as batchpperations but, if desiredjthephosphorous acid which'is continuously produced in the above describedmanher may-"be discharged from the reactor and, succes1-" sively led to dechlorinatin'g and i neutralizing tanks, Where. it'is continuously subjectedi'to e-' chlorination and neutralization by o'ne of the pio;-,. cedures hereinbefore described,

Forexample, the hydrated sodium'phosph'ites, andhemisodium' phosphite may be, advantag "1 ously'produced in a continuous manner by: n tinuously reacting dechlorinated ph'ospl'io us acid with aqueous sodium hydroxidaisuitably cooling" the resulting solution to form a slurry; ofthe desired product and then recoveringthe crystalline product by centrifuging, filteringor in any other suitable manner; v

The various conditions of "operation willnow bew discussed in detail. Q

Inthe production of phosphorous acid in ace. cordance with. the present'invention the hydrolysis" reaction'iscarried (out at a temperature at least equivalent" tolor above the boiling point. ofphosphorus trichloride, but belowth'e temperature at which substantial decomposition of the trichloride into products other thanphosphorous acid occurs. More particularly, the reaction is executed at a temperature substantially in the range of about 76 C. up to about 200 C., the upper limit varying with the concentrations of the reactants.

' In the manufacture of phosphorous acid by the preferred procedure, that is, in a packed tower or equivalent apparatus utilizing a heel of phosphorous acid, the .reaction in the liquid section iseffected at a temperature of from 76 C. up to about 200 C. or preferably 140 C. while the temperature in the vapor section is maintained within the range of 100 C. to about 200 C. Using these operating conditions and substantially stoichiometric amounts of water, it is desirable to supply phosphorus trichloride to the heel at a rate of about 30 to 35 grams per minute. However, the invention is not limited thereto as the P01: feed rate may vary widely so long as about 66% to about 90% and preferably 70% to 80% of the trichloride is hydrolyzed in the liquid section of the tower. Stated in a diiferent manner, it is essential that the P013 feed rate be con trolled so that PC13 entering the vapor section does not exceed about 34% of the trichloride charged, otherwise excessive losses thereof by volatilization and decomposition int-o undesired products will result.

As to the phosphorous acid in the heel or liquid section, it is desirable to limit its concentration to the range of 80% to 98% acid and within this range about 90% to 95% acid is preferred. However, phosphorous acid of lower or higher concentrations may be used if desired, but in no case should the phosphorous acid be of less than 50% strength.

The crude phosphorous acid produced by the method of the instant invention contains about 1% to about 5% by weight of HCl depending upon the HaPOa concentration, temperature and sojourn time. This impurity should be removed, otherwise the acid will slowly decompose on storage, and upon neutralization with alkaline materials, the alkaline phosphites produced will be contaminated with the corresponding chlorides.

The removal of the above impurities may be achieved by heating under a vacuum or, more easily, by aeration or treatment with an inert gas.

For example, substantially complete removal of the HCl is efiected by heating the crude acid .to a temperature ofjfrom 100 C. to 150 C. while under an absolute-pressure of approximately 30 mm. of mercury. This may also be achieved by .aeration of the crude acid for 30 to 60 minutes at .a temperature of from 75 C. to 150 C. with 30 volumes of air per volume of phosphorous acid. Thus, by using the latter method on crude 95% fp'hosphorous acid containing about 2.5% HCl,

this impurity may be reduced to a value of less than 0.01%.

.In the production of sodium phosphites, phosphorous acid and alkaline compounds are reacted tq ether in substantially the proportions theoretically required to produce the desired product. If the anhydrous salts are to be prepared, the reaction is carried out by reacting alkaline materials such as sodium hydroxide, potassium hydroxide, etc. with phosphorous acid in the solid or molten state. They may also be prepared by producing the hydrated salts and then converting the latter to the anhydrous form by heating or extraction with suitable solvents. The hydrated salts are produced by effecting the reaction in an aqueous medium followed by crystallization or by dissolving the anhydrous salts in water and then crystallizing at suitable temperatures. v

The new salt, anhydrous hemisodium phosphite, may be prepared by any of the foregoing methods, but since it crystallizes from solution as an anhydrous salt, it is preferred to carry out the reaction in an aqueous medium and then recover the resulting product by cooling and centrifuging the crystalline slurry.

Where reference is made in the instant specification to phosphorus trichloride and hydrogen chloride (I-ICl), it is to be understood that the corresponding bromine compounds are also contemplated.

Reference is hereby made to application Serial No. 220,583 filed-April 12, 1951, which is a division of the present application.

While the invention has been described with particular reference to specific embodiments, it is to be understood that it is not to be limited thereto, but is to be construed broadly and restricted solely by the scope of the appended claims.

We claim:

1. The method of continuously producing phosphorous acid, which comprises continuously supplying phosphorus trichloride and water, countercurrently to and in contact with each other, to a column reactor containing a vapor section and a liquid section consisting of a heel of at least 50% phosphorous acid and thereby continuously effecting substantially complete hydrolysis of said trichloride to phosphorous acid, and continuously withdrawing said phosphorous acid from said reactor, sald reaction being carried out at a temperature at least equivalent to the boiling point of phosphorus trichloride but below that temperature at which substantial decomposition of said trichloride into products other than phosphorous acid takes place, and said phosphorus trichloride being supplied to said reactor by way of said liquid section at a rate so correlated with the temperature and concentration of the phosphorous acid therein as to effect continuous hydrolysis in the liquid section of at least 66% butsubstantially less than 100% of the trichloride supplied thereto.

2. The method of continuously producin phosphorous acid, which comprises supplying phosphorus trichloride and water, countercurrently to and in contact with each other, to a column reactor containing a vapor section and a liquid section consisting of a heel of at least 50% phosphorous acid and thereby continuously effecting substantially complete hydrolysis of said trichloride to phosphorous acid, and continuously withdrawing said phosphorous acid from said reactor, said reaction being carried out at a temperature at least equivalent to the boiling point of phosphorus trichloride but below that temperature at which substantial decomposition of said trichloride into products other than phosphorous acid takes place, and said trichloride being sup.- plied to said reactor by way of said liquid section at a rate so correlated with the temperature and concentration of the phosphorous acid therein as to effect continuous hydrolysis in the liquid sec.- tion of from 66% to of the phosphorus trichloride supplied thereto. 3. The method of continuously producin phosphorous acid, which comprises continuously supplying phosphorus trichloride and water,

countercurrently to and in contact with each other, to a column reactor containing a vaporsection and a liquid section consisting of a heel of 80% to 9 8% phosphorous acid and thereby continuously effecting substantially complete hydrolysis of said trichloride to phosphorous acid, and continuously withdrawing said phosphorous acid from said reactor, said reaction being carried out at a temperature within the range of 76 C. to 140i C., and said phosphorus trichloride being supplied to said reactor by way of said liquid section at a rate so correlated with the temperature and concentration of the acid therein as to effect continuous hydrolysis in the liquid section of at least 66% but substantially less than 100% of the trichloride supplied thereto.

4. The method of continuously producing phosphorous acid, which comprises continuously supplying phosphorus trichloride and water, countercurrent to and in contact with each other,

to a column reactor containing a vapor section and a liquid section consisting of a heel of 90% to 95% phosphorous acid and thereby continuously effecting substantially complete hydrolysis of said trichloride to phosphorous acid, and continuously withdrawing said phosphorous acid from said reactor, said reaction being carried out at a temperature within the range of 76 C. to 140 C., and said phosphorus trichloride being supplied to said reactor by way of said liquid section at a rate so correlated with the temperature and concentration of the acid therein as to effect continuous hydrolysis in the liquid section of at least 66% but substantially less than 100% of the trichloride supplied thereto.

5. The method of producing phosphorous acid, which comprises continuously introducing phosphorus trichloride into a heel of at least 50% phosphorous acid at a rate efi'ecting hydrolysis of at least 66% but substantially less than 100% of said trichloride into liquid phosphorous acid and a volatilized product including unreacted phosphorus trichloride and hydrogen chloride, continuously contacting said volatilized product with water to substantially completely hydrolyze said unreacted phosphorus trichloride to phosphorous acid and continuously recovering said phosphorous acid, said hydrolysis reactions being carried out at a temperature at least equal to the boiling point of said phosphorus trichloride but below that temperature at which substantial decomposition of said phosphorus trichloride into products other than phosphorous acid takes place.

6. The method defined in claim 5 wherein the water is fed countercurrently to the volatilized product.

7. The method of producing phosphorous acid, which comprises continuously introducing phosphorus trichloride into a heel of from about 80% to about 98% phosphorous acid at a. rate effect,-

10 ing hydrolysis of from about to about of said trichloride into liquid phosphorous acid and a volatilized product including unreacted phosphorus trichloride and hydrogen chloride, continuously contacting said volatilized product with water to substantially completely hydrolyze said unreacted phosphorus trichloride to phos phorous acid and continuously recovering said phosphorous acid, said hydrolysis reactions being carried out at a temperature of from 76 C. to about 200 C.

8. The method of producing phosphorous acid, which comprises continuously introducing phosphorus trichloride" into a heel of from about to about phosphorous acid at a rate effecting hydrolysis of from about 70% to about 80% of said trichloride into liquid phosphorous acid and a volatilized product including unreacted phosphorus trichloride and hydrogen chloride, continuously contacting said volatilized product with water to substantially completely hydrolyze said unreacted phosphorus trichloride to phosphorous acid and continuously recovering said phosphorous acid, said hydrolysis reactions being carried out at a temperature of from 76 C. to about C.

9. The method of producing phosphorous acid, which comprises continuously introducing phosphorus trichloride into a heel of about 90% phosphorous acid at a rate effecting hydrolysis of from about 83% to about 87% of said trichloride into liquid phosphorous acid and a volatilized product including'unreacted phosphorus trichloride and hydrogen chloride, continuously contacting said volatilized product with water to substantially completely hydrolyze said unreacted phosphorus trichloride to phosphorous acid and continuously recovering said phosphorous acid, said hydrolysis reactions being carried out at a temperature of from 76 C. to about 90 C.

JOHN W. LEFFORGE. ROBER'IT'B. HUDSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Mellor--Comprehensive Treatise on Theoretical and Inorganic Chemistry, vol. 8 (1928), Longmans, Green & 60., N. Y.. pages 900-902, 911, 914. 

1. THE METHOD OF CONTINUOUSLY PRODUCING PHOSPHOROUS ACID, WHICH COMPRISES CONTINUOUSLY SUPPLYING PHOSPHORUS TRICHLORIDE AND WATER, COUNTERCURRENTLY TO AND IN CONTACT WITH EACH OTHER, AND A COLUMN REACTOR CONTAINING A VAPOR SECTION AND A LIQUID SECTION CONSISTING OF A HEEL OF AT LEAST 50% PHOSPHOROUS ACID AND THEREBY CONTINUOUSLY EFFECTING SUBSTANTIALLY COMPLETE HYDROLYSIS OF SAID TRICHLORIDE TO PHOSPHOROUS ACID, AND CONTINUOUSLY WITHDRAWING SAID PHOSPHOROUS ACID FROM SAID REACTOR, SAID REACTION BEING CARRIED OUT AT A TEMPERATURE AT LEAST EQUIVALENT TO THE BOILING POINT OF PHOSPHORUS TRICHLORIDE BUT BELOW THAT TEMPERA- 